In recent years, optical information media typified by CDs and DVDs have been widely used as recording media to record large-volume digital data. In general, in a reproduction-only optical information medium, a translucent base, a reflection layer, and a protective layer are laminated sequentially in that order from a light-incident surface side, and in a writable optical information medium, a translucent base, a recording layer, a reflection layer, and a protective layer are laminated sequentially in that order from a light-incident surface side.
The reflection layer serves as an information recording layer in the reproduction-only optical information medium, and the recording layer serves as the information recording layer in the writable optical information medium. When data is read in either of these types of optical information medium, a reproducing laser beam is applied from the translucent base side, and the reflected light thereof is detected. When data is written into the writable optical recording medium, a recording laser beam is applied from the translucent base side, and the chemical state or the physical state of the recording layer is changed by the thermal energy and/or the light energy of the laser beam based on the data to be recorded.
Here, the laser beam applied to the optical information medium is focused with an optical system in such a way that a beam spot having a predetermined radius is formed on the reflection layer or the recording layer. Consequently, if there is a flaw on the surface of the translucent base, the beam spot is not formed properly, and a read error and a write error may occur. A method in which a high-hardness hard coat layer is disposed on the surface of the translucent base has been previously known as a method for preventing occurrence of such a flaw.
In recent years, attempts have been made to increase the numerical aperture (NA) of an object lens used for focusing the recording/reproducing laser beam to on the order of 0.85, and reduce the wavelength λ of the recording/reproducing laser beam to on the order of 400 nm, so as to make the focused spot radius small and, thereby, record large-volume digital data. As a result, recently, a next-generation optical disc format has been made public under the designation of Blu-ray Disc.
Such an increase in NA causes reduction in allowance for warp and inclination, that is, tilt margin, of the optical information medium. Therefore, in order to ensure an adequate tilt margin, the thickness of the translucent base must be decreased. For example, when NA is set at on the order of 0.85 and λ is set at on the order of 400 nm, it is required to decrease the thickness of the translucent base to on the order of 100 μm in order to ensure the adequate tilt margin.
Furthermore, the increase in NA causes reduction in working distance between an object lens and the surface of the optical recording medium. For example, when NA is set at on the order of 0.85, the working distance is decreased to on the order of 100 μm significantly smaller than ever.
However, when the working distance is significantly decreased, there is a very high possibility that the surface of the optical information medium and the object lens or a support supporting the object lens are brought into contact with each other during the rotation of the optical information medium. If such a contact occurs during the rotation of the optical information medium, a fatal flaw may occur on the translucent base surface of the optical information medium. The occurrence of a flaw resulting from such contact can be prevented to some extent by disposing the above-described hard coat layer. However, in the case where the thickness of the translucent base is decreased to on the order of 100 μm, the focused radius of the recording/reproducing laser beam on the translucent base surface is also decreased significantly. Therefore, even a flaw of the size not causing a read error or a write error in known optical information media, e.g., CDs and DVDs, readily cause a read error or a write error. Consequently, a hard coat exhibiting performance higher than ever is required.
The focused radius R of the recording/reproducing laser beam on the translucent base surface is ideally represented by the following expression (1):R=2T tan [sin−1(NA/n)]  (1)(in the expression, T is the thickness (μm) of the translucent base of the optical information medium, NA is the numerical aperture of the object lens in the recording/reproducing device of the optical information medium, and n is the refractive index of the translucent base of the optical information medium).
Therefore, as for a DVD in which NA=0.60 and T=0.6 mm, when n is assumed to be on the order of 1.58, R becomes on the order of 500 μm. On the other hand, as for a system in which NA=0.85 and T=100 μm, R becomes on the order of 130 μm, and the focused spot radius on the translucent base surface becomes significantly small.
Such a significant decrease in focused spot radius on the translucent base surface refers to that the sensitivity to not only a flaw resulting from the contact with the object lens, but also a flaw resulting from handling by the user is extremely enhanced. From this point of view as well, the performance of the hard coat must be significantly improved than ever.
With respect to optical information media, such as a Blu-ray Disc, having a focused spot radius on the translucent base surface significantly smaller than that of a known optical information medium, the abrasion resistance on the translucent base surface must be evaluated by some way in the case where an appropriate hard coat material is selected in the process of development, or in the case where quality control is performed in the process of production. However, under present circumstances, there is no appropriate means therefore other than the method for evaluating an optical information medium described in Japanese Unexamined Patent Application Publication No. 2002-260280 which was applied for a patent by the applicant of the present invention and was laid open.
In general, when the abrasion resistance on paint coatings and resin materials are evaluated, in many cases, sample surfaces are abraded by predetermined abrasion devices, and the amounts of abrasion of test pieces resulting therefrom are quantified by using the amounts of change of various parameters, e.g., mass, thickness, and light transmittance of the test piece. As for optically translucent materials, such as a hard coat layer material of the optical information medium, having relatively high surface hardnesses, it is most appropriate to quantify by using the amount of change in light transmittance or light diffusion. One of the reasons therefore is that the amount of abrasion is not large to the extent capable of being indicated by the amount of change in mass or thickness of the test piece. Specifically, it is generally performed that white parallel light is allowed to incident on the above-described test piece and the haze value thereof is measured.
However, since the evaluation method based on the measurement of the above-described haze value is a method in which transmitted light of the translucent test piece is measured, the method cannot be applied directly to the translucent base surface of the optical information medium.
Another problem occurs in that the determination of appropriate judging criteria of abrasion resistance is difficult in itself with respect to the translucent base surface of the high-recording-density optical information medium, e.g., a Blu-ray Disc. As for optical information media, e.g., CDs and DVDs, which have been already commercialized and become widespread, there are track records of usage by many users. Therefore, the judging criteria of the level of abrasion resistance on the surface of the translucent base required for preventing occurrence of any problem in the daily use can be determined based on those track records. However, as for the significantly high-recording-density optical information media, e.g., a Blu-ray Disc, that is, optical information media in need of high-performance hard coat layer materials, there is no track record of usage by the user. Consequently, it takes some period of time until an adequate track record of usage is established. Therefore, the judging criteria of the level of abrasion resistance required for preventing occurrence of any problem in practice cannot be determined based on the actual track records of usage by users.
Accordingly, it is an object of the present invention to provide a testing method capable of quantifying abrasion resistance on a translucent base surface of a high-recording-density optical information medium simply and in a form of reflecting an actual application environment, and to provide judging criteria appropriate for the testing method.